Revert "net/http: do not discard body content when closing it within request handlers"

Revert "net/http: do not discard body content when closing it within request handlers"

This reverts commit cb0d9980f5721715ebb73dd2e580eaa11c2ddee2.

Reason for revert: the old behavior seems to be relied on by current
users, e.g.
https://github.com/connectrpc/connect-go/blob/cb2e11fb88c9a61804043355a619c12d4a30a1a5/protocol_connect.go#L837.

For #75933

Change-Id: I996280238e5c70a8d760a0b31e3a13c6a44b8616
Reviewed-on: https://go-review.googlesource.com/c/go/+/721761
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>